I think it's more about the fact that there is a major infrastructure for the production of gasoline that is well well established. One way to derive better efficiency from gasoline is through direct injection, which many of the big players did.
Diesel is still very efficient. Let's not forget that there are others producing diesel engines that are not violating emissions regulations.
We still don't know all the info behind what VW did and for what reasons. Although I'm sure it will boil down $$.
They certainly had the capability to meet emissions when the smog gear was hooked up, but perhaps this could not be sustained in the long run? Was it just to get a few more MPG?
Here's the kicker.. Let say you have one of these effected vehicles. Now you get a call from the dealer saying you have to bring in your car in to get "fixed". The "fix" implies either - You are are going to get less mpg OR your emissions equipment is not going to last as long OR BOTH.
To which my response is - No thanks she is running just fine! I will skip the "fix".
The fact of the matter is that these VW diesels make up less than 1% of the cars out there and the higher NOx emissions they are producing does not really have an impact here in the US. Although that may not be the case in Europe.